1. Field of the Invention
The present invention relates generally to a poppet valve for an internal combustion engine, and more particularly to a composite lightweight engine poppet valve having a valve head and at least a part of the stem constructed of a titanium intermetallic material with other parts of the poppet valve manufactured from a different material or materials capable of operating at high temperatures and in a corrosive environment.
2. Description of the Related Art
It is well known in the industry that engine poppet valves operate at relatively high temperatures and in corrosive environments. It is further known that exhaust valves operate under more severe conditions than intake valves. The exhaust valves used in diesel and leaded fuel applications are typically considered heavy duty applications and operate in more severe conditions than do valves in other types of internal combustion engines. Engine poppet valves must be able to withstand even the most severe of these conditions, be functional and have a long life.
Efforts are constantly being made to improve the construction, design, and manufacturing techniques of engine poppet valves. These efforts include, but are not limited to, making the valve lighter in weight, more economical to manufacture, and more durable. Lighter poppet valves reduce valve train friction, and reduce noise vibration. Lightweight poppet valves can be manufactured more cost effectively due to less material employed in the valve. In addition, engine poppet valves can be designed with specific chemical and mechanical properties in mind to withstand the harsh temperature and environmental conditions to which engine poppet valves are subjected. Some of these desired chemical and mechanical properties include, but are not limited to, excellent sulfidation resistance, good hot hardness, sufficient oxidation resistance, optimum thermal resistance, and low thermal expansion.
The term “lightweight” as employed herein is intended to refer to the physical characteristics of the engine poppet valve and its components that mare the engine poppet valve light in weight. The term “lightweight” is also meant to include but not be limited to an ultralight engine poppet valve as that term is defined in U.S. Pat. No. 5,413,073 which is owned by the assignee of the present invention and is hereby incorporated by reference. The assignee of the present invention also owns U.S. Pat. Nos. 5,453,314 and 6,263,849, which are both hereby incorporated by reference. Suppliers of automotive engine poppet valves are continually searching for better materials to make better poppet valves. Intermetallic materials, like titanium aluminide (TiAl), offer the superb qualities of being light weight and heat resistant. Casting TiAl in long, thin shapes, like engine poppet valve stems, is difficult. The TiAl castings often must be hot isostatically pressed (HIP) to reduce porosity. Obtaining high quality, low porosity TiAl cast engine valves is expensive and time consuming. Moreover, despite the superb qualities of TiAl, this material is very brittle, crack sensitive, and lacks wear resistance when in contact with carburized steels, nitrided steels, or high carbon powder metals, i.e., the conventional materials used in valve assemblies. TiAl and other intermetallics are difficult to weld materials due to their brittleness. The intermetallics generally offer less than two percent elongation, whereas conventional materials like steel are characterized by about fifteen to about thirty percent elongation at room temperature and at atmospheric pressure.
While it is possible to fusion weld intermetallics, the welding technique usually includes very high temperature preheats, about 1000 degrees C., and very high temperature postheats. In addition, fusion welds in intermetallics typically are permitted to cool, in a controlled manner, in the fixture after the postheating period. Following postheating, welds in intermetallics receive a post weld stress relief and/or a post weld heat treatment. In the fabrication of engine poppet valves, this approach lacks economic viability and is not practical for high volume manufacturing.
Thus, there is still a need to make improvements in the manufacturing technique and design of a composite lightweight engine poppet valve. The improved engine poppet valve according to the present invention is constructed in an arrangement that allows the valve head and at least part of the valve stem of the engine poppet valve to be manufactured from a titanium intermetallic material and to be attached to other selected parts of the engine poppet valve which have been manufactured from other conventional materials.